Black spot compensation apparatus



May 28, 1940. R. ANDRIEU 2,202,511

BLACK SPOT COMPENSATION APPARATUS Filed April 28, 1937 2 Sheets-Sheet 132 lll/lllll IIIIIIIII AIAAAIAAAA AAA AAA AA vvnvvvvvv v 14451617 I IEIEJEJKI 00000 KNEE r0000 o o o o EEIEIMzsV o o o o--29 "KW/ fil I F2%%m [y I INVENTOR ROBERT ANDRIEU BY Z/W'IM/ ATTORN EY May 28, 1940.-

SCANNING MOSA/C R. ANDRIEU BLACK SPOT COMPENSATION AXPARATUS Filed April28, 1937 2 Sheets-Sheet 2 AMPLIFIER T0 7H4 MSM/ 7' 7' [R .SCAA/N/A/GVIDEO MON/TOR CONTROL 708E AMPLIFIER TUBE 7' UB5 INVENTOR ROBERT ANDRIEUATTORNEY Patented May 28, 1940 S TAT E UNITE PATENT @Ft'lQE BLACK SPQ'liCQMPENSATION APPARATUS ticn, of Germany Application April 28, 1937,Serial No. 139,356 in Germany April 28, 1936 8 Claims.

My invention relates broadly to apparatus for the compensation ofundesired signals in electrical circuits, and more particularly toapparatus of that form which is for use in compensating for so-calledblack spot phenomenon generally associated with television scanners ofthe photo-mosaic type.

Cathode ray picture scanners of the type in' which a mosaic or signalplate produces a charge pattern of the subject to be televised are ingeneral subject to a spurious or undesired phenomenon generallyidentified as black spot phenomenon. This stray signal is additive innature, that is to say, it also exists when the picture to betransmitted to a remote point consists of a uniformly white or greycolor in addition to the situation in which the picture comprises anumber of light and dark contrasts.

A number of theories have been proposed accounting for the fact that themosaic is subjected to dark or light portions which are not present inthe optical image itself. There seem to be several distinct causes towhich this phenomenon may be attributed, and the first of these is thatof a non-uniformity in the beam velocity of the cathode ray beam. This,of course, amounts to a modulation of the beam itself and as a result,its interpretation or measurement as it may be called of the chargevalues on various portions of the photoelectric mosaic is subject to thesame variations as the variations in the beam velocity. As a result, atthe receiver a dark spot or region appears on the screen a portion ofwhich at least may be attributable to nonuniformity in the beam velocityeither at the transmitter or at the receiver.

The second possible cause for so-called dark spot results from the factthat in a number of transmitting tubes, there is provided a first andsecond anode which are used in conjunction with the photoelectricmosaic. The second anode is usually used to collect secondary electronemission and to focus the beam. The photoelectric mosaic is of necessityat an angle with respect to the major axis of the tube, and it is usualto place the second anode so that its major axis is co-incident withthat of the tube; the result is that the various points on thephotoelectric mosaic are unequally distant from and with respect to thesecond anode. Accordingly, this inequality in influence is acontributing factor to black spot production.

Again, due both to the unequal charge pattern and the unequal strengthexerted by the field from the second anode, there is developed acrossthe photoelectric mosaic itself a variable field. This variable fieldinfluences the path of the electrons emitted by the photoelectric mosaicor the secondary emission occasioned by the cathode ray beam bombardmentthereof and, to some extent, this unequal and variable field across thephotoelectric mosaic adds to the dark spot phenomenon.

A fourth cause is the influence of either electric or magnetic fieldswhich are generated externally to the tube itself. This may be caused byspurious oscillations in the neigborhood of the apparatus, may be causedby magnetization of objects in the neighborhood of the apparatus, and asa result, these fields may be either constant or variable in nature,and. as a general rule they are variable. As a result these influencesin fields, whose influence on the apparatus is uncontrolled insofar asthe mosaic itself is concerned, contribute their shape to the undesiredblack spot or region produced.

Again, it is also possible that the actual response of the photoelectricmaterial on the photoelectric mosaic to light values of equal intensitymay not be exactly the same over a period of time. This may be dueeither to original unequal characteristics of the definite photoelectricparticles or it may be due to an unequal dissipation on the strength ofthe particle which may possibly be caused by subjecting one portion ofthe screen to an unusually high electron bombardment or it may be due toa variable response of the photoelectric elements over a period of time.However, the short non-linearity of response may well contribute itsshare towards the so-called black spot phenomenon.

It is customary, or considered good practice at present, to compensatefor this undesired stray signal or black spot signal by providing foreach picture co-ordinate a. voltage which is variable in proportion intime to the progress of the cathode ray beam in the picture co-ordinate,and for this purpose there is provided a voltage which will varyaccording to one sinuous period over the length of the picture or frameco-ordinate and another voltage which is variable according to twosinuous periods over the length of the picture co-ordinate. Theamplitude and phase of each of these three components may be adjustable,but with this arrangement it is neces sary to adjust twelve variablequantities for a definite distribution of the black spot signal if thelatter is to be properly compensated. This sort of compensated apparatushowever has the undesirable drawback that it is impossible to as varythe size of the compensating signal for a definite point of the picturewithout incidentally and simultaneously effecting a change in thecompensating quantity at other points in the picture.

It is, therefore, an object of my invention to provide apparatus andways and means by which a black spot portion at one part of a picturemay be properly compensated without incidentally affecting thecompensation of the black spot region at other points in the pictureand, broadly, speaking, I do this by dividing the picture into aplurality of distinct regions and providing means which will vary thecompensation for each of these regions, the means being separate,distinct, and independent of each other. By means of my apparatus thereis set up in the cathode ray scanner a compensating signal which is afunction of the adjustment of a particular device which is adapted tocompensate for one region independently of each of the othercompensators which provide an adjustment in their separate and distinctregions of the picture.

My invention will best be understood by reference to the drawings, inwhich Fig. 1' is an embodiment of my compensation generator,

Fig. 2 is a front view of the sectionalizer along line AB, I

Fig. 3 is an explanatory diagram,

Fig. 4 shows a'compensating control board,

Fig. 5 is an explanatory diagram,

Fig. 6 is an embodiment of my invention, and

Fig. '7 is a schematic explanatory diagram.

Referring to Fig. 1, an evacuated vessel ill has "means shown as l I forgenerating a cathode ray beam, For purposes of simplicity, these meansare shown schematically therein. However, they include a cathode, acontrol grid, and a first anode; The cathode ray pencil is subject tothe action of two fields which are at right angles to each other, saidfields being indicated by" the arrows l2 and I3, and these fields varyin intensity with the horizontal and Vertical scanning frequencies withrespect to time. Located"at"the end of the tube remote from the electronpencil generating means is a sectionalized bank of electrodes i4, i5,i6, ll etc., and in this view there is shown a cross-section of four ofthe electrodes. However, it will be appreciated that this is a viewthrough one angle an'd'that the entire bank of electrodes thereinillustrated comprise sixteen in number. This number has been chosen forpurposes of convenience and not for purposes of limitation. Apotentiometer is energized by a source of potential SI and the positiveend of the potentiometer isjoined through a resistor 32 to the secondanode of the tube 3!, this anode being common to allzof theaforementioned electrodes.- Each of the electrodes is joined to thepotentiometer 35 by a conductor, and the position of the contact isvariable so that the bias on the electrodes may be'varied at any desiredtime. The anode 3! acts as a collector for secondary emission fromthe'electrodes I 4 through 29;

Referring to Fig. 2, there is shown the relative relationships inposition of the electrodes i l through 29 'as'seen' from the front ofthe tube 10.

Referring to Fig. 3, four of the electrodes are shown as M through H andare indicated for purposesof clarity in dotted section. This figureillustrates what may be the relative distribution of shading or darkspot on the received picture or the monitored picture from the trans-Initter. This view shows a continuous section of dark spot or shading,but it will be appreciated that the shading may be distributed onvarious spots as well as concentrated in a section.

The operation of the device may be understood with reference to theforegoing explained Figs. 1, 2 and 3. Assuming that the stray or darkspot signal isdistributed as shown in Fig. 3, if on the scanning screenof the transmitted tube the cathode ray beam begins to scan the firstline of the picture, the cathode ray pencil in the tube Ill which movessynchronously with that in the transmitter begins to move first over theelectrode l4, and it will impinge on the electrode marked in dottedlines in Fig. 3 as M. The stray signal of this district is comparativelypowerful so that during the time when the scanning cathode ray beamsweeps the district of the pictureidentified as M, a proportionatelypowerful compensating signal must be produced. This is effectd byjoining the electrode 14 of the tube ill to a potential which iscomparatively negative relative to that of secondary anode 3! so thatthrough a resistance 32 a continually strong secondary emission currentwill be caused to flow.

The fall of potential occasioned across this resistance may be used tocompensate for the dark spot in the picture in the transmitting tube by,for instance, controlling the operating level of the video amplifier,controlling the modula-.

tion level of the transmitter, .etc., these being well known to thoseskilled in the television.

art.

As the scanning beam reaches the district identified as Iii, the cathoderay beam in tube It] passes over at the same time to an electrode M ifThe dark spot is not quite as pronounced in this district and,accordingly, a lower compensating potential will be needed. According:

ly, the electrode i5 is renderedless negative with respect to thesecondary emission collecting an ode 3i than was the electrode l4; -Hence,.there will be a lower fall of potential across resistor 32 and thispotential is used in'the manner indi cated with respect 'to thepotential developed the electrode MI The black spot in the section i6 iseven less than thatin section l5 and I4 and, accordingly, the negativepotential lim pressed on It with respect to the anode 3! is less thanthat of either electrodes M and Hi. Therefore, the compensating signalwill be less than the one furnished by those two electrodes, and

a careful, adjustment of these biasingpotentials willeliminate"independently the dark spotfron each of these sectio'ns of thepicture. .In the re gion covered by electrode flit will beseen with ireference to Fig. 3 that there is no darl; spot present'and hence, therewill be no biason the electrode H with respect to the second anode 3!.Hence, no compensating potential is produced since no compensation isnecessary.

The steady change of the stray signal over the length of individuallinesupon the scanning,

screen is taken into practical consideration by 65 =5= that the cathoderay pencil, as already pointed out, has in the plane A, B, acomparatively large cross-section with the result that in spite of thefinite diiference of theelectrode potential, the

ceding electrode. In the same manner, a steady change of the straysignal along the time-base and. picture coordinate at right angles tothe direction of the line is insured in that also in this co-ordinatethe cathode-ray pencil in tube It changes steadily from one electrode tothe next.

An arrangement of the kind hereinbefore described is particularly easilyoperable if the control knobs whereby the potentials at the variouselectrodes may be regulated are disposed in a plane in a similar way asthe corresponding districts of the scanning screen. This is illustratedin Fig. 4 where the knobs i i to 29" which correspond to the variouselectrodes numbered M to 29 are placed adjacent and below one another(1. e. juxtaposed and infraposed) like the electrodes or the screendistricts or subdivisions.

' What may also be pointed out is that the compensation of the straysignal is not merely accomplishable by subtraction of a supplementalsignal as furnished from a tube as shown in Fig. 1, but that the shapeof the additional signal as a function of the time may also be chosen insuch a way that upon addition of this supplemental signal to picture(video) and stray signals, the disturbing action is eliminated with theexception of a signal component which is of constant size throughout thelength of a frame.

This scheme shall be explained by reference to Fig. 5 which by way ofexample shows the light intensity distribution as caused only by thestray signal over a frame line. The compensation method hereinbeforedisclosed results in a compensatory signal which is altered as theordimates of the surface F1 presenting oblique shading, while in thepresence of a corresponding adjustment of the electrode potential also asupplemental signal could be generated which varies with the ordinatesof the surface F2 shown with vertical shading, so that by addition tovideo and stray signal there will just be neutralized the distributionof light intensity occasioned by the stray signal.

Referring to Fig. 6, there is shown an arrangement for automaticallyadjusting the value of the biasing potentials for the electrodes Itthrough 29. The tube I0 having smaller electrodes to that shown in thetube illustrated in Fig. 1 will have its action explained in conjunctionwith the scanning mosaic and a further arrangement. The scanning mosaic5!! is joined to a video amplifier 5! which in turn controls amonitoring or other reproducing tube 52. Hence, the picture developed bythe scanning apparatus will be reproduced by the cathode ray tube 52,having a screen 53 on which the picture is reproduced. In front of thisscreen and subject to the light values of the particular portionsthereof are a set of individual photoelectric cells 54, 55, 5B, and 51,and one cell is provided to be used in conjunction with each of theelectrodes l4 through 29. Now, for instance, if there is a dark shadingon the section of the picture corresponding to H the photocell 5? willrespond, its impulses will be amplified by the amplifier 60 which isjoined to electromagnetic means 6! for attracting the moving arm of thepotentiometer 62 which ordinarily may be restrained by spring 63. Thephotocell may be arranged to control the amplifier in a negativedirection, that is to say, that the potential drop may bias the grid ofthe amplifying tubes in such a direction that a small photocell currentrenders the amplifying tube more positive so that a greater outputresults and a large photocell current biases the tube negatively so thata smaller output current results. Hence, it will be obvious that thesmaller the light value in the region of photocell 51, the greater willbe the pull of the normally restrained moving arm 62 and, accordingly,the electrode I1 will be biased more negatively with respect to thesecondary anode 3i, and a charge compensating potential will bedeveloped across the resistor 32 which, it will be seen, is arranged soas to control the operating level of the video amplifier 5|.

Since television transmitters are of fairly large size, it is notprohibitive that sixteen distinct photocells be furnished and sixteenamplifiers operating in conjunction therewith for the automaticcompensation of the black spot signal. It will be appreciated thatvarious modifications of the scheme shown in Fig. 6 may be availed ofas, for instance, a selecting system as shown in Alexanderson et al U.S. Patent 1,787,851 might be used to control the biasing potential oneach of the individual electrodes M through 29 in progressive steps as,for instance, a definite relay might close for absolute dark or black,and this relay might control a definite negative potential which isimpressed on one of the electrodes [4 through 29 of the tube l0 since itwould be quite possible to adequately compensate with a range of sayfive voltages corresponding to black, light black, grey, light grey, andsome intermediate shade, five distinct potentials would eliminate aconstant revision in bias that would take place as the arm moved acrossthe potentiometer. While no specific figure has been shown covering thisarrangement, it will be appreciated that it is clearly within myconcept.

Referring to Fig. '7, there is shown schematically the operation of thesystem wherein the scanning tube joins the video amplifier which in turnfurnishes potentials to a monitored tube and to the transmitter, themonitor tube energizing the control tube which in turn controls theoperating level of the video amplifier.

What I claim is:

1. Apparatus for developing corrective potentials for eliminating blackspot efiects in television scanners comprising means for developing acathode ray beam, means for deflecting said beam, a plurality ofelectrodes each representative of a discrete section of the opticalView, the cathode ray beam deflecting means being adapted to sweep thebeam sequentially across each of the discrete elements, means forbiasing said elements, and load means associated with said elements fordeveloping corrective potentials.

2. Apparatus for developing corrective potentials for eliminating blackspot effects in television scanners comprising means for developing acathode ray beam, means for deflecting said beam, a plurality ofelectrodes each representative of a discrete section of the opticalview, the cathode ray beam deflecting means being adapted to sweep thebeam sequentially across each of the discrete elements, commonpotentiometer means for biasing said discrete electrodes, means forpositioning the moving elements on said potentiometer from a potentialdeveloped from the monitored image to be transmitted and load meansassociated with said elements for developing corrective potentials.

3. Apparatus for developing corrective potentials for eliminating blackspot efiects in television scanners comprising means for developing acathode ray beam, means for deflecting said to sweep the beamsequentially across each of the discrete elements, means for'biasing-'said elements under the control of discrete sections of the monitoredimage of the image to be transmitted, load means connected to saidelements for-developing corrective potentials, and anodic means forcollecting the secondary emission from each of the discrete elements,means for biasing said elements under the control of discrete sectionsof the monitored image of the image to be transmitted, load meansconnectedto said elements for developingcorrective potentials, andanodic means forpollecting the secondary emission from the saidelectrodes, said anodic means being connected to the load means.

5. The method of compensating for black spot efiects in televisionscanning apparatus which comprises the steps of reproducing theimagedeveloped by scanning as a monitor image, dividing the produced monitorimage into a plurality of sectional areas, developing from the monitorimage a potential representative of the background levelof each of saidsectional areas in sequence, utilizing said potentials to developproportional corrective signal potentials and utibeam; a plurality of"electrodes/each r'epresenta--= tive of a discrete section of theopticalview; the cathode ray beam defiecting means being adapted llizing S8id- 0OITeCfiVe signal potentials .to substantially compensateforv blacklspot' efiects.

6.'The method in accordance with. claim 5',

wherein thesectional areas into which the monitor image is divided aresubstantially equalini area; i

7. Apparatus in accordance with claim 1, wherein a monitor image of theview to be transmittedis developed and separate photocell means eachassociated with one of the electrode means are positioned adjacent saidmonitor image, and

whereinthe variable biasing means are controlled by the signalsdeveloped within said photocell means.

8. Apparatus for developing corrective potentials forcompensating forblackrspot efiects in television'rscanners comprising means for de-'veloping-a monitor optical view 'of the signals developed within ascanner, a plurality of photoelectric cells each representing a discretesectional-area of the monitor image, said photocells being positionedadjacent the means for developing the monitor imageand adapted to beenergized thereby, means for developing :a cathode ray beam, a pluralityof discrete electrodes each representing a sectional area of the monitorimage, potentiometer means electrically connect-' ed to each of saiddiscrete electrodes, means under the control of each of saidphotoelectric means for determining the position of the variable sectionof said potentiometers, means for deflecting the cathode ray beamwhereby said beam sequentially impinges on each of the discreteelectrode means, and load means associated with said discrete electrodemeans for developing corrective potentials.

' ROBERT ANDRIEU.

